Gases are easily compressible, and liquids are not.

Pressure increases as matter is compressed.

The particles of a liquid are farther away from one another than the particles in a solid. However, the particles of a liquid are still packed quite closely together. Thus, a liquid cannot be compressed and has fixed volume. The pressure of liquids can be calculated bye height x density x gravity.

The particles of a gas have a lot more space between them as compared to particles of liquids or solids. The large space between the particles of a gas allows the gas to be easily compressed when pressure is applied, hence gas has no fixed volume.

Particles in a gas are well separated with no regular arrangement, vibrate and move freely at high speeds. Particles in a liquid are close together with no regular arrangement, vibrate, move about, and slide past each other. Particles in a solid are tightly packed, usually in a regular pattern, vibrate (jiggle) but generally do not move from place to place.

The compressibility:

Solids

Generally smaller (they resist deformation more, because compressibility is defined with an explicit negative sign) than for liquids by ~ 3X. However, there is overlap because other factors are also important.

The compressibility: “K = -(dV/dP)/V” varies with temperature, pressure, and other variables, so it

is quite difficult to measure.

Liquids

For each atmosphere increase in pressure, the volume of water would decrease 46.4 parts per million. The compressibility k is the reciprocal of the Bulk modulus, B.

Compressibility, k (Pa-1 x 10-11) for

Carbon disulfide is 93, Ethyl alcohol is 110, Glycerine is 21, Mercury is 3.7, Water is 45.8

Compressibility, k (Atm-1 x 10-6) for

Carbon disulfide is 94, Ethyl alcohol is 111, Glycerine is 21

Mercury is 3.8, Water is 46.4

Gases

The gas compressibility factor (Z) describes the difference between ideal and actual behaviour of a gas. The Pressure/Volume/Temperature relationship for a non-ideal gas becomes

PV=ZnRT

Where Z is the compressibility factor. In fact, Z should be written as Z(P,T), as it is a function of both pressure and temperature.

For pressures close to atmospheric, it is common to use a value of Z set to 1.0, and similarly, when the pressure is controlled very close to design setpoint, it is usually adequate to use the fixed design compressibility.

If, however, the pressure and temperature can vary during normal operating conditions, then there can be a signficant variation in the compressibility factor for a gas.

The pressure:

Pressure is calculated by dividing the force by the area over which it is applied.

pressure = force / area (P = F/A)

The SI unit of pressure is the pascal (Pa). (1Pa =1N/m²)

Liquids in a gravitational field, like all fluids, exert pressure on the sides of a container as well as on anything within the liquid itself. This pressure is transmitted in all directions and increases with depth. In the study of fluid dynamics, liquids are often treated as incompressible, especially when studying incompressible flow.

p = g / z

p = the density of the liquid (assumed constant)

g = gravity

z = the depth of the point below the surface.

*This formula assumes that the pressure at the free surface is zero, and that surface tension effects may be neglected.

Pressure can be increased by either increasing the size of the force or reducing the area over which the force acts.

The surface of the earth is at the bottom of an atmospheric sea. The standard atmospheric pressure is measured in various units:

1 atmosphere = 760 mmHg = 29.92 inHg = 14.7 lb/in2 = 101.3 KPa

The fundamental SI unit of pressure is the Pascal (Pa), but it is a small unit so kPa is the most common direct pressure unit for atmospheric pressure. Since the static fluid pressure is dependent only upon density and depth, choosing a liquid of standard density like mercury or water allows you to express the pressure in units of height or depth, e.g., mmHg or inches of water.